Airbag device

ABSTRACT

An airbag of an airbag device for a driver seat has a front panel and a back panel. In the airbag having undergone spontaneous inflation, when a size between a securing position of the airbag to a retainer member and the most protruding portion of the airbag with respect to the securing position is set as 1, the most protruding portion by the back panel is located at a position spaced from the securing position by 0.5 or greater, a diameter of the inflated airbag of an opposing portion to an inner surface of the wheel is 0.6 to 1.2 times as large as an inner diameter of the wheel, and a diameter of the inflated airbag of a joint portion of the front panel and the back panel is greater than the diameter of the inflated airbag of the opposing portion to the inner surface of the wheel.

BACKGROUND

1. Technical Field

The disclosure relates to an airbag device for a driver seat.

2. Description of the Background Art

Japanese Patent Application Laid-Open Publication No. 2007-15478discloses an airbag of which an upper fabric and a lower fabric cut intocircular shapes are stitched each other at outer peripheral edgesthereof to form a bag shape. The airbag is configured to inflate in anoblate spheroid shape towards an occupant-side with respect to asteering wheel by gas generated from an inflator, thereby holding theoccupant.

Japanese Patent Application Laid-Open Publication No. 2013-14176discloses an airbag having an extension portion arranged at least at oneside edge in a width direction of a vehicle and deployable to extendtowards a rear side of the vehicle. The airbag is accommodated in acenter pad, which is arranged to have a non-rotational structure withrespect to a steering wheel so that the extension portion can bedeployed at a constant position at any steering angle of the steeringwheel.

SUMMARY Technical Problem

When an obstacle collides obliquely forward with a vehicle, an occupantis moved obliquely forward due to the inertia force occurring upon thecollision.

In Japanese Patent Application Laid-Open Publication No. 2007-15478, themovement of the airbag having undergone the oblate spheroid inflation isrestrained by the steering wheel. At this state, in order to hold theoccupant, who intends to move obliquely forward, by the airbag, it isnecessary to increase a diameter of the inflated airbag. To this end, itis necessary to increase a volume of the airbag.

Also, according to Japanese Patent Application Laid-Open Publication No.2013-14176, the airbag has the extension portion, so that it isnecessary to increase the volume as much as that.

In order to increase the capacity of the airbag, it is necessary to usean inflator having a large amount of gas generation, which in turnincreases the cost.

It is therefore an object of the disclosure to provide an airbag for adriver seat configured to hold an occupant who is moving obliquelyforward, while avoiding an increase in a capacity of the airbag as muchas possible.

Solution to Problem

In order to achieve the above object, a first aspect of the disclosureprovides an airbag device that is attachable to a steering wheelincluding a wheel hub and an annular wheel provided around the wheelhub. The airbag device includes an inflator that generates gas; anairbag that is inflatable by the gas generated by the inflator andincludes a front panel, which includes at least one fabric and iscapable of restraining an occupant upon inflation under emergency of avehicle, and a back panel, which includes at least one fabric and isprovided at a side closer to the wheel hub than the front panel; and aretainer member that is secured to the wheel hub at a state where theinflator and the airbag are secured. In the airbag having undergonespontaneous inflation without being attached to the steering wheel, whena size between a securing position of the airbag to the retainer memberand the most protruding portion of the airbag with respect to thesecuring position is set to 1, the most protruding portion by the backpanel is located at a position spaced from the securing position by 0.5or greater, a diameter of the inflated airbag of an opposing portion toan inner surface of the wheel is 0.6 to 1.2 times as large as an innerdiameter of the wheel, and a diameter of the inflated airbag of a jointportion of the front panel and the back panel is greater than thediameter of the inflated airbag of the opposing portion to the innersurface of the wheel.

A second aspect of the disclosure is the airbag device of the firstaspect wherein in the airbag having undergone spontaneous inflationwithout being attached to the steering wheel, when the size between thesecuring position of the airbag to the retainer member and the mostprotruding portion of the airbag with respect to the securing positionis set to 1, a central position of the airbag is located at a positionspaced from the securing position by 0.5 or greater.

A third aspect of the disclosure is the airbag device of the secondaspect wherein mass of the at least one fabric per unit area, which isto be included in the front panel, is greater than mass of the at leastone fabric per unit area, which is to be included in the back panel.

A fourth aspect of the disclosure is the airbag device of the second orthird aspect wherein the front panel includes a mass body attached to atleast a part of the front panel.

Advantageous Effects

According to the airbag device of the first aspect, in the airbag havingundergone spontaneous inflation, since the diameter of the inflatedairbag of the opposing portion to the inner surface of the wheel is 0.6to 1.2 times as large as the inner diameter of the wheel, when theairbag secured to the retainer member is inflated, the airbag isdifficult to contact the inner surface of the wheel with high force.Also, when the size between the securing position and the mostprotruding portion of the airbag with respect to the securing positionis set as 1, the joint portion of the front panel and the back panel islocated at a position spaced by 0.5 or greater. Also, since the diameterof the inflated airbag of the joint portion is greater than the diameterof the inflated airbag of the opposing portion to the inner surface ofthe wheel, an inflation shape of the back panel becomes greater towardsthe occupant-side across the wheel. Therefore, a space can be easilyformed between a surface of the wheel facing the occupant-side and theairbag.

For this reason, the airbag can swing about a securing place to theretainer member. When the occupant is contacted to the airbag, eventhough a moving direction of the occupant is oblique with respect to afront-rear direction of the vehicle, the airbag can be obliquely movedin the moving direction of the occupant. Thereby, it is possible to holdthe occupant who is moving obliquely forward, while avoiding an increasein a capacity of the airbag as much as possible.

According to the second aspect, since the central position of the airbagis positioned between the retainer member and the occupant or located ata position close to the occupant-side, the airbag can be more easilymoved about the securing place to the retainer member.

According to the third aspect, the mass of the at least one fabric perunit area, which is to be included in the front panel, is greater thanthe mass of the at least one fabric per unit area, which is to beincluded in the back panel. Thereby, it is possible to set the centralposition of the airbag at a side close to the occupant.

According to the fourth aspect, it is possible to increase the mass ofthe front panel by the mass body, so that it is possible to set thecentral position of the airbag at a side closer to the occupant.

These and other objects, features, aspects and advantages of the presentdisclosure will become more apparent from the following detaileddescription of the present disclosure when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view depicting an airbag device inaccordance with an illustrative embodiment.

FIG. 2 illustrates an inflated state of an airbag.

FIG. 3 depicts examples of panels configuring the airbag.

FIG. 4 illustrates a state where an occupant moving in an obliquelyforward direction of a vehicle is contacted to the airbag.

FIG. 5 depicts a positional relation of respective parts of the airbaghaving undergone spontaneous inflation.

FIG. 6 depicts a positional relation of respective parts of a steeringwheel.

FIG. 7 illustrates an airbag in accordance with a modified embodiment.

FIG. 8 illustrates an airbag in accordance with another modifiedembodiment.

FIG. 9 illustrates an airbag in accordance with still another modifiedembodiment.

FIG. 10 depicts a modified embodiment of the panel configuring theairbag.

FIG. 11 depicts size examples of various airbags.

FIG. 12 depicts changes in an angular velocity of a head when the headis contacted to a variety of airbags.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, an airbag device of an illustrative embodiment will bedescribed. FIG. 1 is an exploded perspective view depicting an airbagdevice 10, and FIG. 2 illustrates an inflated state of an airbag 40.Meanwhile, in FIG. 2 and the like, a wheel 22 of a steering wheel 20 anda central part of a wheel hub 26 are shown.

The airbag device 10 is configured so that it can be attached to thesteering wheel 20 in a vehicle.

The steering wheel 20 is provided to steer the vehicle and has a wheel22, spokes 24 and a wheel hub 26.

The wheel 22 has an annular shape and is a part to which a steeringforce is to be applied from an occupant (a driver). The wheel hub 26 isconfigured so that it can be coupled to a tip portion of a steeringshaft extending from the front of a driver seat towards the occupant inthe vehicle. The spokes 24 extend from an inner surface of the wheel 22towards a center of the wheel and are coupled to the wheel hub 26. Here,the three spokes 24 are provided. However, the two spokes may also beprovided, for example. When the wheel 22 is rotated, the rotationalmovement is transmitted to the steering shaft through the spokes 24 andthe wheel hub 26.

The steering wheel 20 is usually configured by an armature and a resincovering part configured to cover the armature. The armature is formedof metal and has a configuration where a part configuring the wheel hub26, a part configuring inner cores of the spokes 24 and an annular partconfiguring an inner core of the wheel 22 are integrated. The resincovering part is configured to cover the part configuring the spokes 24and the part configuring the wheel 22 of the armature. A part of theresin covering part, which configures the wheel 22, is also referred toas a rim.

The wheel hub 26 is configured to secure and accommodate the airbagdevice 10. Here, the wheel hub 26 is formed with an accommodating recessportion 26 h that opens towards an occupant-side. Also, the wheel hub 26is formed therein with engaging protruding portions 23 capable ofengaging and securing the airbag device 10. The airbag device 10 isaccommodated and secured in the accommodating recess portion 26 h at astate where the engaging protruding portions 23 are engaged and securedto the airbag device 10.

In a direction of a central axis of the wheel 22, the wheel hub 26 isarranged at a position closer to the steering shaft than the wheel 22,i.e., at a position distant from the occupant.

The airbag device 10 has an inflator 30, an airbag 40 and a retainermember 50. Here, the airbag device 10 has a cover 60 configured to closethe opening of the accommodating recess portion 26 h, too.

The inflator 30 is configured to generate gas. Here, the inflator 30 hasan inflator main body 32 having a short cylinder shape and attachingflanges 34 formed on an outer periphery of the inflator main body 32. Inthe inflator main body 32, an ignition device, a gas-forming agent andthe like are incorporated. When a detection signal from a shockdetection unit is received upon collision of the vehicle, the ignitiondevice ignites the gas-forming agent. Thereby, the gas-forming agent iscombusted, so that the gas generated by the combustion is supplied intothe airbag 40. Thereby, the airbag 40 is inflated and deployed towardsthe driver. The attaching flanges 34 have a plate shape extending fromthe outer periphery of the inflator main body 32 so that outerperipheral edges thereof form a rectangular shape, and four cornerportions thereof are formed with securing holes 34 h.

The airbag 40 has a bag shape formed of fabric and the like, and isfolded to be accommodated in the accommodating recess portion 26 h. Theairbag is configured to inflate by the gas that is to be generated bythe inflator 30.

More specifically, FIG. 3 depicts examples of panels configuring theairbag 40. The airbag 40 has a front panel 42 and a back panel 44.

The front panel 42 is provided at a part at which the occupant can berestrained upon inflation of the airbag 40 under emergency of thevehicle. That is, the front panel 42 is a part that is provided at theoccupant-side at a state where the airbag 40 has undergone inflationwith being attached to the steering wheel 20. Here, the front panel 42consists of one fabric and has a circular shape. As the fabric, a wovenfabric, a fabric of which a woven fabric is applied with a coating agentsuch as silicone, and the like are used. The front panel 42 may consistof at least one fabric or a plurality of fabrics partially or entirelysuperimposed. Meanwhile, in FIG. 3, stitch lines M are shown at an outerperiphery of the front panel 42 and an outer periphery of the back panel44.

The back panel 44 is provided at a side closer to the retainer member 50(the wheel hub 26) than the front panel 42. That is, the back panel 44is a part that is provided at an opposite side to the occupant-side atthe state where the airbag 40 has undergone inflation with beingattached to the steering wheel 20. Here, the back panel 44 consists ofone fabric and has a circular shape. As the fabric, a woven fabric, afabric of which a woven fabric is applied with a coating agent such assilicone, and the like are used. Also, the back panel 44 may consist ofat least one fabric or a plurality of fabrics partially or entirelysuperimposed.

The back panel 44 is formed with an attaching hole 44 h. A part (a parthaving a gas exhaust port) of the inflator main body 32 is arranged inthe airbag 40 through the attaching hole 44 h. Also, the back panel 44is formed with a plurality of (four, here) screw insertion holes 44 haaround the attaching hole 44 h. Further, the back panel 44 is formedwith vent holes 44 hb for discharging the gas after the airbag inflationbetween the attaching hole 44 h and a peripheral edge portion of theback panel 44.

The peripheral edge portion of the front panel 42 and the peripheraledge portion of the back panel 44 are joined by the stitching, forexample, so that the airbag 40 is formed. The joined portion of theperipheral edge portion of the front panel 42 and the peripheral edgeportion of the back panel 44 is a joint portion 41.

In the meantime, the shapes of the panels 42, 44 of the airbag 40 andthe inflation shape of the airbag 40 are not limited to the aboveexamples. A modified embodiment thereof will be described later. Also,the panels 42, 44 may be configured by a combination of a plurality offabrics. An example where the panel 44 is configured by a combination ofa plurality of fabrics will be described later. Also, the inflationshape of the airbag 40 can be regulated by a tether belt arranged toconnect insides of the airbag. FIG. 5 depicts an example where a tetherbelt 48 is arranged to connect a periphery of the attaching hole 44 hand a part of the occupant-side of the airbag 40.

Returning to FIG. 1 and FIG. 2, the retainer member 50 is configured sothat it can be secured to the wheel hub 26 at a state where the inflator30 and the airbag 40 are secured.

More specifically, the retainer member 50 is a member having a plateshape and made of metal and the like, and is formed at its centralportion with an inflator arranging hole 52 h capable of arranging theinflator main body 32 therein. The retainer member 50 is formed withscrew insertion holes 52 ha around the inflator arranging hole 52 h. Ata state where the inflator main body 32 is arranged in the inflatorarranging hole 52 h and a part (a part having a gas exhaust port) of theinflator main body 32 is arranged in the airbag 40 through the attachinghole 44 h, screws 58 protruding from an internal retainer plate arrangedin the inflator 30 are inserted into the screw insertion holes 44 ha,the screw insertion holes 52 ha and the securing holes 34 h. Nuts 59 arescrewed and fastened to the screws 58, so that the inflator 30 issecured to the retainer member 50. Also, the peripheral edge of theattaching hole 44 h of the airbag 40 is interposed between the internalretainer plate and the retainer member 50, so that the airbag 40 issecured to the retainer member 50.

Also, the retainer member 50 is formed with receive-side engaging parts56 capable of engaging and securing the securing protruding portions 23provided to stand from the wheel hub 26. The securing protrudingportions 23 are fitted and secured to the receive-side engaging parts56, so that the retainer member 50 is secured to the wheel hub 26. Atthis state, the inflator 30 and the airbag 40 secured to the retainermember 50 are also accommodated in the accommodating recess portion 26 hof the wheel hub 26.

At this state, the retainer member 50 is positioned at a back of theaccommodating recess portion 26 h. For this reason, a securing positionQ of the airbag 40 to the retainer member 50 is located at an oppositeside to the occupant than a position of the wheel 22 in the direction ofthe central axis of the wheel 22.

Also, the retainer member 50 is formed with cover securing pieces 57protruding towards the occupant-side.

The cover 60 is a member formed of resin and the like, and has a covermain body 62 configured to cover the opening of the accommodating recessportion 26 h and an occupant-side of the folded airbag 40, and has aperipheral wall 64 protruding from a back side of the cover main body 62and configured to cover a periphery of the folded airbag 40. The coversecuring pieces 57 of the retainer member 50 are secured to theperipheral wall 64 by screwing, riveting, an engaging and securingstructure, and the like. Thereby, the cover 60 is also secured to theretainer member 50.

At a state where the retainer member 50 is secured to the wheel hub 26,the peripheral wall 64 of the cover 60 is configured to cover the airbag40 folded in the accommodating recess portion 26 h, i.e., the cover mainbody 62 is configured to cover the opening of the accommodating recessportion 26 h. A part of the cover 60 facing the folded airbag 40 isformed with a tear-line for easy tearing. When the airbag 40 isinflated, the airbag 40 tears the cover main body 62 along the tear-line(not shown), so that an opening for inflation of the airbag 40 isformed. The airbag 40 can be inflated from the steering wheel 20 towardsthe occupant-side through the opening.

At a state where the airbag 40 is inflated from the steering wheel 20,the airbag 40 is deployed towards the occupant-side along the centralaxis of the inflator main body 32. For this reason, when the occupant ismoved towards the front of the vehicle along a front-rear direction ofthe vehicle due to the inertia force generated upon the collision of thevehicle, such as the front collision of the vehicle, the occupant isreceived at the central part of the airbag 40.

In the meantime, when an obstacle collides obliquely forward with thevehicle, for example, the occupant is moved obliquely forward due to theinertia force generated upon the collision. In this case, when theairbag 40 keeps a state where it is deployed towards the occupant-sidealong the central axis of the inflator main body 32, the occupant iscontacted to the peripheral part of the airbag 40. In order toeffectively hold the occupant, the airbag 40 is preferably configured toreceive the occupant at a position as close as possible to the centralpart of the airbag.

Therefore, here, the airbag 40 is configured to swing about a securingplace to the retainer member 50. Thereby, as shown in FIG. 4, when theoccupant P intends to move obliquely forward, the occupant P iscontacted to the airbag 40 in the obliquely forward direction of thevehicle. Then, the airbag 40 is pushed by the occupant P, so that theairbag swings about the securing place to the retainer member 50. Forthis reason, the airbag 40 can receive the occupant P at a position asclose as possible to the central part thereof. Thereby, it is possibleto effectively hold the occupant P, who intends to move in the obliquelyforward direction of the vehicle, by the airbag 40.

A following configuration is adopted so that the airbag 40 can easilyswing about the securing place to the retainer member 50.

First, the wheel 22 is provided at a position closer to theoccupant-side than the securing place of the airbag 40 to the retainermember 50. For this reason, the airbag 40 is inflated towards theoccupant-side through the inside of the wheel 22. At a state where theairbag 40 is inflated into a bag shape, since the wheel 22 exists aroundthe securing place, the wheel 22 operates to prevent the airbag 40 fromswinging. Therefore, a following configuration is adopted so as toprevent the interference between the inflated airbag 40 and the wheel 22as much as possible.

That is, as shown in FIG. 5, the airbag 40 having undergone spontaneousinflation without being attached to the steering wheel 20 is assumed.Here, the spontaneous inflation of the airbag 40 means a state where theairbag 40 has undergone full spontaneous inflation by the gas generatedfrom the inflator 30. For this reason, as an actual inflation shape ofthe airbag 40, an inflation shape considering the impetus of the gasfrom the inflator 30 is assumed. First of all, following conditions arepreferably satisfied even at a state where the vent holes are closed, sothat the air is filled and the airbag 40 is thus inflated.

At the above inflation state, a diameter of the inflated airbag 40 of anopposing portion to an inner surface of the wheel 22 is set as a size A.That is, as shown in FIG. 6, a distance (on the basis of a place (here,a center of the section) at which an inner diameter of the wheel 22 issmallest) of the wheel 22 to the securing position Q of the airbag 40 tothe retainer member 50 is denoted as L1. As shown in FIG. 5, at a statewhere the airbag 40 has undergone spontaneous inflation, a diameter ofthe inflated airbag 40 at a position spaced from the securing position Qby the distance L1 is set as the size A. Also, as shown in FIG. 6, aninner diameter (the smallest inner diameter) of the wheel 22 is denotedas B.

In this case, the size A (the diameter of the inflated airbag 40) is setto a size of 0.6 to 1.2 times as large as the size (the inner diameterof the wheel 22) B. More preferably, the size A (the diameter of theinflated airbag 40) is set to a size of 0.6 to 1.0 times as large as thesize (the inner diameter of the wheel 22) B.

Also, at the inflation state, a size between the securing position Q andthe most protruding portion of the airbag 40 with respect to thesecuring position Q is denoted as L2, and a size between the securingposition Q and the joint portion 41 of the front panel 42 and the backpanel 44 is denoted as L3. At the state where the airbag 40 hasundergone spontaneous inflation, a diameter of the inflated airbag 40 ofthe joint portion 41 of the front panel 42 and the back panel 44 is setas a size C.

In this case, when L3≧0.5×L2, i.e., the size L2 is 1, the mostprotruding portion (i.e., the joint portion 41) by the back panel 44 islocated at a position spaced from the securing position Q by 0.5 orgreater. Also, the size C (the diameter of the inflated airbag 40 of thejoint portion 41) is set to be greater than the size A (the diameter ofthe inflated airbag 40) of the opposing portion to the inner surface ofthe wheel 22.

According to the above example, usually, the back panel 44 becomesgradually greater towards the occupant-side. Thus, the joint portion 41is arranged at a position distant from the securing position Q as far aspossible and the inflation is made to make the diameter of the inflatedairbag 40 of the joint portion 41 as great as possible. For this reason,it is possible to make the diameter of the back panel 44 as small aspossible at the inside of the wheel 22 and to set a gravity center G ofthe airbag 40 at a position close to the occupant P. Thereby, the airbag40 can easily swing about the securing place to the retainer member 50.

In the meantime, usually, when it is assumed that the back panel 44becomes gradually greater towards the occupant-side, as the position ofthe joint portion 41 of the airbag 40 is more distant from the securingposition Q to the retainer member 50, an inclination angle α of the backpanel 44 relative to an extension direction (an extension plane of theretainer member 50) of the securing position Q shown in FIG. 5 becomesgreater. As the inclination angle α becomes greater, a space β between asurface of the wheel 22 facing towards the occupant-side and the airbag40 upon the attachment to the steering wheel 20 is greater, so that theairbag 40 can further swing with respect to the external force.

In the meantime, the part (the joint portion 41), which is a target ofthe size L3 (the diameter of the inflated airbag 40 of the joint portion41), has a linear shape, in FIG. 5. However, the joint portion 41 (thestitch portion) may have an undulating shape, depending on anoverlapping shape of a yarn direction of fabric of the front panel 42and a yarn direction of fabric of the back panel 44. In this case, anaverage value of respective sizes of a maximum size part and a minimumsize part of the undulating shape from the securing position Q isdetermined as the size to the joint portion 41.

Thereby, when the airbag 40 secured to the retainer member 50 isinflated, the airbag 40 is difficult to contact the inner surface of thewheel 22 with high force. For this reason, the airbag 40 can swing aboutthe securing place to the retainer member 50. When the occupant iscontacted to the airbag 40, even though the moving direction of theoccupant is oblique with respect to the front-rear direction of thevehicle, the airbag 40 can move so that it is inclined in the movingdirection of the occupant. Thereby, it is possible to effectively holdthe occupant who is moving obliquely forward, while avoiding an increasein a capacity of the airbag 40.

In the meantime, the size A and the inclination angle α can beimplemented by adjusting the maximum diameter of the inflated airbag 40,providing the tether belt 48 in the airbag 40, changing the panel shapeof the back panel shown in FIG. 3, and the like.

Regarding a method of adjusting the inclination angle α, the front panel42 shown in FIG. 3 has a circular shape, for example. Also, the backpanel 44 has a shape where recess portions 44 a having a notched shapeof which a width is gradually narrowed from an outer periphery-sidetowards a center are formed at parts (here, two parts facing each otherwith the center being interposed therebetween) of a circular outerperiphery. When side portions of each recess portion 44 a, which faceeach other, are joined by the stitching and the like, the back panel 44has a conical shape. When the peripheral edge portion of the back panel44 having the conical shape and the peripheral edge portion of the frontpanel 42 are joined by the stitching and the like, it is possible toobtain the airbag 40 capable of inflating into a shape where the size L3to the joint portion 41 is located at a position spaced from thesecuring position Q by 0.5 or greater when the size L2 between thesecuring position Q of the airbag 40 to the retainer member 50 and themost protruding portion of the airbag 40 with respect to the securingposition Q is set as 1, as shown in FIG. 5.

Meanwhile, in order to largely set the inclination angle α (in order toenlarge the most protruding portion obtained by the back panel) in FIG.5, a central angle γ (here, the central angle γ is defined on the basisof the stitching place) of the recess portion 44 a shown in FIG. 3 ispreferably made to be large. To the contrary, in order to small set theinclination angle α, the central angle γ of the recess portion 44 a ispreferably made to be small. Thereby, it is possible to easily adjustthe inclination angle α of the back panel 44.

Meanwhile, in FIG. 3, the width of the recess portion 44 a defines anarrowing (expanding) degree of the back panel 44. For this reason, itis possible to appropriately adjust the inflation shape of the backpanel 44 and to implement an airbag 40B (refer to FIG. 7) of which thediameter of the inflated airbag 40 stepwise changes by appropriatelyadjusting the shape of both edge portions of the recess portion 44 a.

Also, according to the airbag 40B of a modified embodiment shown in FIG.7, in the airbag 40B having undergone spontaneous inflation withoutbeing attached to the steering wheel 20, a portion (a portion morespaced from the securing position Q towards the occupant-side than thesize L1) of the airbag 40B, which is located at a position closer to theoccupant-side than the opposing portion to the inner surface of thewheel 22, includes a portion that is to be inflated more rapidly than aportion (a portion close to the securing position Q from a positionspaced from the securing position Q towards the occupant-side by thesize L1) closer to the retainer member 50-side than the opposing portionto the inner surface of the wheel 22.

That is, in FIG. 7, a portion of the airbag 40B spaced from the securingposition Q towards the occupant-side beyond the size L1 includes aportion of which an inclination angle α2 is smaller than a maximuminclination angle α1 (an inclination angle relative to the extensionplane of the retainer member 50) of a portion (here, a portion of thesecuring position Q) of the airbag 40B closer to the securing position Qthan the position spaced from the securing position Q towards theoccupant-side by the size L1.

In other words, in the example of FIG. 7, the inclination angle of theairbag 40B gradually decreases towards the protruding directionfront-back of the wheel 22 in the direction of the central axis.

For this reason, it is possible to enable the airbag 40B to easily swingabout the securing place to the retainer member 50 by making thediameter size A of the opposing portion of the airbag 40B to the innersurface of the wheel 22 as small as possible. At the same time, theairbag 40B can be enabled to inflate as large as possible at a positionacross the wheel 22 towards the occupant-side, so that it is possible tomore securely hold the occupant who is moving obliquely forward.

In the examples of FIG. 5, FIG. 7, FIG. 8 and FIG. 9, a portion of theairbag closer to the retainer member 50 than the wheel 22 may beconfigured to gradually greater towards the retainer member 50.

Also, the gravity center of the airbag 40 is preferably located at aposition close to the occupant so that the airbag 40 can more easilyswing about the securing place to the retainer member 50.

That is, as shown in FIG. 5 and the like, in the airbag having undergonespontaneous inflation without being attached to the steering wheel 20,the size between the securing position Q of the airbag 40 to theretainer member 50 and the most protruding portion of the airbag 40 withrespect to the securing position Q is denoted as L2. A size between aposition of the gravity center G of the airbag 40 and the securingposition Q is denoted as L4. When L₄≧0.5×L2, i.e., L2=1, the gravitycenter G is preferably located at a position spaced by 0.5 or greater.

Thereby, the gravity center G of the airbag 40 is set to a positionclose to the occupant, and the airbag 40 can easily swing about thesecuring position Q to the retainer member 50, so that it is possible tomore securely hold the occupant who is moving obliquely forward.

In order to set the gravity center G of the airbag 40 to a positionclose to the occupant, it is possible to set the gravity center G to aposition close to the occupant by contriving the inflation shape of theairbag 40. The airbag 40 shown in FIG. 5 is set so that it widelyinflates at the occupant-side and narrowly inflates at the steeringwheel 20-side. Thereby, it is possible to set the gravity center G to aposition close to the occupant. Also, it is possible to set the gravitycenter G to a position close to the occupant by adopting a variety ofother configurations (refer to modified embodiments of FIG. 8 and FIG.9, which will be described later).

For example, like an airbag 40C of a modified embodiment shown in FIG.8, mass of a front panel 42C corresponding to the front panel 42 is madeto be greater than mass of a back panel 44C corresponding to the backpanel 44, so that it is possible to set the gravity center G to aposition close to the occupant.

To this end, mass of at least one fabric 42Ca per unit area, which is tobe included in the front panel 42C, is preferably made to be greaterthan mass of at least one fabric 44Ca per unit area, which is to beincluded in the back panel 44C.

Specifically, a density of the fabric 42Ca of the front panel 42C ispreferably made to be greater than a density of the fabric 44Ca of theback panel 44C. The densities of the fabrics 42Ca, 44Ca can be adjustedby changing the number of yarns per unit area, a thickness of yarn, andthe like.

Also, the masses per unit area of the fabric 42Ca of the front panel 42Cand the fabric 44Ca of the back panel 44C can be adjusted by changingwhether or not to coat (silicone coating, for example) the fabric 42Caof the front panel 42C and the fabric 44Ca of the back panel 44C,thicknesses thereof, and the like, too.

Also, like an airbag 40D of a modified embodiment shown in FIG. 9, it ispossible to set the gravity center G to a position close to the occupantby attaching a mass body 45D to a front panel 42D corresponding to thefront panel 42.

That is, the airbag 40D has the front panel 42D and the same back panel44D as the above illustrative embodiment.

The front panel 42D has a circular fabric 42Da and a mass body 45Dattached to the fabric 42Da. The mass body 45D is a woven fabric, afabric of which a woven fabric is applied with a coating agent such assilicone, and the like, like the fabric 42Da, and is attached to themass body 45D by the stitching, for example. Here, the mass body 45D hasa sheet shape smaller than the fabric 42Da. The two mass bodies 45D areattached to front and back sides of a central portion of the fabric42Da. The mass body 45D may have a sheet shape of which a spreadingdegree is substantially the same as the fabric 42Da. By thisconfiguration, it is possible to set the gravity center G of the airbag40D to a position close to the occupant by increasing the mass of thefront panel 42D with the mass body 45D.

In the meantime, the back panel 44 shown in FIG. 3 may be configured bya plurality of panels. For instance, in an example of FIG. 10, a backpanel 44′ is configured by a combination of two divided panels 44 aa, 44ab, which are to be overlapped each other at the peripheral edge of theattaching hole 44 h, and is formed with the same recess portions 44 a′as the recess portions 44.

For the airbag 40 of the above illustrative embodiment and the airbag ofthe modified embodiment shown in FIG. 10, the sizes of the respectiveportions of the airbag 40, the position of the gravity center G and thelike were analyzed by CAE (computer aided engineering). The results areshown in FIG. 11. In the meantime, as the CAE analysis software, MADYMO(TASS International Co., Ltd.) was used. Also, as the steering wheel 20,a steering wheel of which the inner diameter size B of the wheel 22 is314.8 mm and the size L1 between the securing position Q and the wheel22 is 43.2 mm was assumed.

At the spontaneous inflation states of a variety of embodiments, a ratio(A/B) of the size A, which is the diameter of the inflated airbag 40 ofthe opposing portion to the inner surface of the wheel 22, and the innerdiameter size B of the wheel 22, and a ratio (L3/L2) of the size L2between the securing position Q of the airbag 40 to the retainer member50 and the most protruding portion of the airbag 40 with respect to thesecuring position Q and the size L3 to the joint portion 41 of the frontpanel 42 and the back panel 44 are shown in FIG. 11.

In the meantime, regarding the shape of the airbag 40, the analysis wasperformed for a total of six types including the oblate spheroid shapeof the related art and the five embodiments in which the back panel 44of the illustrative embodiment shown in FIG. 5 has a conical shape. Inthe embodiments 1 to 3, the inclination angle α is sequentially changedso that it stepwise increases, the tether belt having a predeterminedlength is set and the diameter of the inflated airbag 40 of the jointportion 41 is respectively adjusted so that all the capacities aresubstantially the same. In the embodiment 4, the tether belt 48 is notset on the basis of the embodiment 3, and in the embodiment 5, thelength of the tether belt 48 is shortened on the basis of the embodiment3.

Comparing the oblate spheroid shape and the embodiments 1 to 3, as shownin FIG. 11, when the back panel 44 is made to have a conical shape, theratio of the size A (the diameter of the inflated airbag 40) to theinner diameter size B of the wheel 22 is decreased. Also, as the ratioof the size L3 to the size L2 increases, the inclination angle αincreases. As a result, the space β between the wheel surface facing theoccupant and the bag is increased, so that the airbag 40 can be movedmore easily in conformity to the movement of the occupant P.

Also, as shown in the embodiments 4 and 5 of FIG. 11, it can be seenthat the ratio of the size A (the diameter of the inflated airbag 40) tothe size B (the inner diameter of the wheel 22) is changed depending onwhether or not the tether belt 48 and the length thereof.

If the length of the tether belt 48 is too shortened, the ratio of thesize A (the diameter of the inflated airbag 40) to the size B (the innerdiameter of the wheel 22) increases and the central position tends tocome close to the retainer member 50-side from the occupant-side.Therefore, even when the airbag 40 is configured by the combination ofthe panels shown in FIG. 3 or 10, it is necessary to appropriatelyconsider the diameter of the inflated airbag 40, the length of thetether belt 48, and the like.

Also, for the respective embodiments of FIG. 11, the CAE analysis wasperformed to examine whether it is possible to effectively hold theoccupant P who is moving obliquely forward. The evaluation was performedby the angular velocity at which a head of the occupant P swings aboutan axis connecting a neck and a crown of the head after the head isreceived on the airbag having the oblate spheroid shape of the relatedart and the airbag 40 of the disclosure.

The analysis result is shown in FIG. 12. In FIG. 12, a curve V1indicates the airbag having the oblate spheroid shape, a curve V2indicates the embodiment 1, a curve V3 indicates the embodiment 2, acurve V4 indicates the embodiment 3, a curve V5 indicates the embodiment4 and a curve V6 indicates the embodiment 5.

As shown with the curves V1 to V4 of FIG. 12, it can be seen that theangular velocity of the head is small and the head is effectively heldin the embodiments. In particular, as the ratio (L3/L2) and the ratio(A/B) are greater (that is, as the inclination angle α increases), itcan be seen that the angular velocity of the head decreases and the headcan be effectively held.

Also, as shown with the curve V5, even when the tether belt 48 is notprovided, it is possible to decrease the angular velocity, and as shownwith the curve V6, even when the ratio of the size A (the diameter ofthe inflated airbag 40) of the airbag 40 to the inner diameter B of thewheel 22 increases, the good influence on the angular velocity can bemade.

From the above analysis result, it can be said that when the ratio ofthe size A (the diameter of the inflated airbag 40) of the airbag 40 tothe inner diameter B of the wheel 22 is set to 0.6 to 1.2 times, it iseffective to set the size L3 to the joint portion 41 of the front panel42 and the back panel 44 to a position spaced from the securing positionQ by 0.5 or greater.

In the meantime, the respective configurations described in theillustrative embodiment and the modified embodiments can beappropriately combined inasmuch as the configurations are mutuallyconsistent.

For example, the modified embodiment of FIG. 7 and the modifiedembodiment of FIG. 8 may be combined, and the modified embodiment shownin FIG. 7 or FIG. 8 and the modified embodiment of FIG. 9 or FIG. 10 maybe combined.

Although the disclosure has been described in detail, the abovedescriptions are just exemplary in all aspects and the disclosure is notlimited thereto. A variety of modified embodiments not exemplified canbe made without departing from the scope of the disclosure.

1. An airbag device that is attachable to a steering wheel comprising awheel hub and an annular wheel provided around the wheel hub, the airbagdevice comprising: an inflator that generates gas; an airbag that isinflatable by the gas and comprises a front panel, which comprises atleast one fabric and is capable of restraining an occupant uponinflation under emergency of a vehicle, and a back panel, whichcomprises at least one fabric and is provided at a side closer to thewheel hub than the front panel; and a retainer member that is secured tothe wheel hub at a state where the inflator and the airbag are secured,wherein in the airbag having undergone spontaneous inflation withoutbeing attached to the steering wheel, when a size between a securingposition of the airbag to the retainer member and the most protrudingportion of the airbag with respect to the securing position is set as 1,the most protruding portion by the back panel is located at a positionspaced from the securing position by 0.5 or greater, a diameter of theinflated airbag of an opposing portion to an inner surface of the wheelis 0.6 to 1.2 times as large as an inner diameter of the wheel, and adiameter of the inflated airbag of a joint portion of the front paneland the back panel is greater than the diameter of the inflated airbagof the opposing portion to the inner surface of the wheel.
 2. The airbagdevice according to claim 1, wherein in the airbag having undergonespontaneous inflation without being attached to the steering wheel, whenthe size between the securing position of the airbag to the retainermember and the most protruding portion of the airbag with respect to thesecuring position is set as 1, a gravity center position of the airbagis located at a position spaced from the securing position by 0.5 orgreater.
 3. The airbag device according to claim 2, wherein mass of theat least one fabric per unit area, which is to be included in the frontpanel, is greater than mass of the at least one fabric per unit area,which is to be included in the back panel.
 4. The airbag deviceaccording to claim 2, wherein the front panel comprises a mass bodyattached to at least a part of the front panel.